A method for duplicating proximal chromosomal segments using AB chromosomes was reported by Carlson and Curtis in 1986 (Can. J. Genet. Cytol. 28:1034). Two duplications were constructed, one for chromosome 9 and one for chromosome 3. In both cases, overlapping AB chromosomes were utilized. For example, the 9B from TB-9Sb and the 9B from TB-9La were combined to produce plants containing 9BSb 9BSb 9BLa 9BLa in place of two normal 9's. Preliminary cytological data indicated that this type of duplication is relatively stable in inheritance.
A genetic test for stability of the chromosome 9 duplication is reported here. Stability depends on the formation of homomorphic bivalents in meiosis: 9BSb 9BSb and 9BLa 9BLa. If heteromorphic bivalents form or if a quadrivalent forms, crossing over between 9BSb and 9BLa could occur. Crossing over would produce a normal 9 and break down the duplication.
To test for formation of a crossover 9, duplication homozygotes carrying the Wx marker were crossed as female to a wx tester. If the crossover 9 was not produced in the female, all progeny should contain 9(wx) 9BSb (Wx) 9BLa(Wx). However, if crossing over occurred between 9BSb and 9BLa, some progeny should carry a normal 9(Wx). The constitution of progeny was checked by germinating forty-nine kernels and crossing the plants as male parents to a wx tester. One ear per male parent was classified for Wx vs. wx phenotypes. Forty-six of the forty-nine plants tested gave Wx frequencies below 50% (average 29% Wx. Range 15-42% Wx). Three of the plants gave Wx rates of 50% or more (52%, 59%, 66%). The frequency expected in the presence of a crossover 9(Wx) is at least 5017c. Therefore, forty-six plants must have carried the (noncrossover) duplication chromosomes. Selection against the duplication by pollen competition gave the reduced Wx frequencies. The three plants that gave 50% or more Wx probably contained a crossover 9(Wx) (in addition, they may have contained an extra 9BSb or 9BLa or crossover 9, depending on chromosomal segregation patterns.) An alternative possibility to the presence of a crossover 9 in the aberrant crosses is the presence of extra chromosomes through meiotic nondisjunction of 9BSb or 9BL, . This would mean that plants which gave aberrant ratios contained either 9(c wx) 9BSb(Wx) 9BSb(Wx) 9BL,(Wx) or 9 (c wx) 9BSI)(Wx) 9BLa(Wx) 9BLa(Wx). Both types of plants might transmit Wx at high frequency due to the extra chromosome carrying Wx (chromosome number=22). However, Wx progeny from a 22 chromosome plant should usually contain 21 chromosomes. When Wx kernels on the three aberrant ears were sampled (ten per ear) all had 20 chromosomes. Consequently, crossing over rather than meiotic nondisjunction appears to be the cause of ears with a high Wx frequency. This finding makes utilization of the proximal "double AB" duplications more difficult, since periodic re-testing of stocks will be needed to insure their constitution.
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